1. Field of the Invention
This invention relates to methods and apparatus used in the design and manufacture of surfboards, sailboards or similar aquatic boards, referred to generically herein as “board” or “boards.”
2. Description of the Related Art
Surfboards and sailboards are similar in shape and basic structure—the board typically has a high strength exterior skin covering that protects and is supported by very low-density material in the interior core; in construction, moldable plastic is used for the compound curvatures and sharp trailing edge contours conducive to a low-drag hydrodynamic shape; the board's primary strength is derived a woven fabric, made from high-strength glass, carbon or aramid fiber, that is imbedded in the plastic to form a fiber-reinforced plastic or plastic composite skin.
The composite skin, which is very thin, can be reinforced with specially manufactured high-density PVC sheet foam, end-grain balsa or honeycomb core materials to form a “structural sandwich” or “cored composite.” The stiff, lightweight core material, used as a substrate to separate the high-strength composite layers on either side, creates a fundamentally different structure—the sandwiched core delivers the stiffness and rigidity of much thicker material, but at a fraction of the weight, and provides the impact resistance and compressive strength the very thin layers of plastic composite lack.
The composite core materials and reinforcing fabrics impart a high degree of stiffness and very high strength but, unlike the weaker plastics and foamed plastics with which they are combined, have a limited capacity to conform to compound curvature (i.e., a surface that curves in two directions at once). Where the curvature is severe, divisions are necessary to prevent structural defects such as wrinkles in the reinforcing fabric, or the breakage and/or failure of the core material to conform to the required shape. Since a break in the continuity of either material causes a large reduction in strength, the placement of a division—usually referred as a joint or seam—is critical to the overall structural integrity of the board.
Currently, with molded methods of production, or in custom “one-off” manufacture (i.e., when the board is fabricated by hand) a joint or seam is required to accommodate the sharp curvature at the board's perimeter edge or “rail”—this division creates a number of seemingly unrelated but very serious problems, which increase manufacturing costs and seriously compromise the board's potential strength.
In prior art molded manufacture, for example, the sharp curvature around the board's perimeter edge compounds a number of very basic drawbacks inherent in the mold's concave configuration itself—the resulting structural problems and high manufacturing costs make the mold, and the board structure requiring fabrication in the mold, fundamentally unsuited for board production. The problems begin with the mold's inward turning surface—saturated by hand, the resin naturally tends to flow out of the reinforcing fabric and pools in the concave cavity of the mold; the mold's sharp edge contours then create a dam that makes it very difficult for the squeegee to completely remove the excess—the result is a weak, heavy resin-rich skin. In areas of severe concave curvature, wrinkles in the reinforcing fabric easily occur, and are difficult or impossible to remove—pulling the fabric taut tends to lift it from the surface of the mold; pushing on the fabric is analogous to pushing on a string, and causes wrinkles to (re)appear.
To minimize the above problems, in the prior art the mold is divided into top and bottom halves—with the relatively flat and shallow surface the fabric is easily aligned and much of the excess resin can be successfully removed—the placement of the part-line, however, is in the worst position possible—at the board's exposed perimeter edge. Because the division of the mold also breaks the continuity of the high-strength fiber, the mold-seam on the finished board has only a fraction of the strength of material where the fiber is fully intact. The design of the joint is then compromised by the limitations of the mold's concave surface—the mold-seam is far stronger and reinforcement is much more effective when applied to the interior of the joint, which becomes completely inaccessible once the mold is closed. The mold-seam is typically reinforced after removal from the mold; this adds weight to the already resin-rich skin, and sufficient rework to negate much of the labor-saving advantage.
The difficulty molding the board's interior foam core then raises production costs further still—because the expansion of plastic foam involves heat (e.g., polyurethane foams undergo an exothermic reaction; steam is required to expand EPS “bead” foams) there is both an expansion and a very slight cooling contraction cycle in the foam; the slight cooling contraction makes it very difficult to pre-mold the board's interior foam core to sufficiently tight tolerances to eliminate potential voids between the surface of the foam and the mold—when the expansion occurs in the mold, the cooling begins before the foam has fully hardened, and often causes poor adhesion or an inconsistent skin-to-interior core bond.
To reduce the problem, in the prior art the foam is contained in an extremely strong mold and the very high outward pressure generated by the foam's expansion is used to attain adhesion and an adequate bond. Drawbacks include the high cost of the mold—which typically has steel reinforcing jigs attached and is held in a hydraulic press or by other mechanical means to prevent buckling, separating or failure under the high pressure of the expansion—and the higher density of the foam and added weight.
The additional problem is that the plastic composite is thin and bendable, and the resin generally shrinks between five and six percent as it cures. The direction of shrinkage is primarily into the fiber and against the surface of the mold, where it is held in place by the perfect vacuum that develops as the resin hardens and cures. Because the two halves must eventually meet at a precise point around the perimeter, the function of the mold is to stabilize the laminate, and prevent distortion or shrinkage of the resin from creating a mismatch between the two opposing sides—the skin must then be fully cured and receive the support of additional material (ordinarily provided by the bond between the two opposing sides and the board's interior core) before it can be removed from the mold. The order of application is a major problem: the least stable and longest curing material (i.e., the composite skin) is applied to the mold first, quickly curing foams or pre-molded interior core structures are added later—this lengthens the mold-cycle and causes very slow production.
a. Molded Methods of Production
With excess weight, high-capital costs, and lack of any competitive advantage in terms of price, the only manufacture of molded fiberglass skin/polyurethane foam core surfboards occurred in the early nineteen-sixties, soon after the introduction of polyurethane foam, and were derisively referred to as “pop-outs” due to their structural inferiority. The commercial production of molded hollow boards was attempted in the early nineteen seventies, but was also very brief—absent the foam, the lack of an effective joint between the board's top and bottom sides (see, e.g., U.S. Pat. No. 3,514,798 to Ellis) caused the mold-seam at the perimeter to split open with relatively modest impact; with higher impact often detaching the skin from the interior support structure, the damage was difficult or impossible to repair.
Reviewing prior art clearly shows the structural defects and compromises caused by the concave configuration of prior art female molds. U.S. Pat. No. 3,802,010 to Smith, for example, suggests that the mold-seam at the board's perimeter can be eliminated by dividing a conventional female mold into right and left halves, and laying the saturated fiberglass fabric into the mold in a single sheet. According to the invention, the centerline division means that there are no joints along either side or rail where the board is subject to the greatest beating during use.
What is completely ignored is the fact that the board's outline around the perimeter is roughly twenty percent longer than the straight line along the axis of symmetry—if the part-line is placed at the shortest distance between the nose and tail of the board, the fiberglass must elongate a total of ten percent per side to cover the perimeter of the mold, while maintaining its original length at the center. Since fiberglass is not elastic, ten percent of the fabric will be excess, and will appear as folds and wrinkles in direct proportion to the differential in length.
The sharp folds in the reinforcing fabric create voids if subsequent layers are applied on top—this precludes the possibility of adding fiberglass layers or the use of any composite core material at all, or using these materials to create a bonding/reinforcing flange between the two opposing sides. The joint is formed by pouring a very thick layer of adhesive into a concave depression in the foam core, creating a very weak and heavy mold-seam between the opposing sides. The mold's deep internal cavity and lack of access makes it impossible to accurately trim and create an overlapping joint in the fabric at the perimeter of the mold, and also prevents the defects from being properly repaired. The invention suggests trading the well-known structural problems caused by the relatively shallow concave cavity of the conventional female mold, for the much larger defects of a very deep one.
The closely related U.S. Pat. No. 4,383,955 to Rubio et al. specifically identifies a number of the more obvious problems outlined above, and teaches a conventional solution: to improve access, the right- and left-hand mold configuration to Smith is given an extra division that turns it into quarters—with the four relatively flat mold surfaces, the fiberglass fabric can be successfully applied to the mold without wrinkles, moreover, with the accessible mold surface ordinary steps such as polishing, prepping and applying release agents to the mold become possible so that the board can subsequently be removed. From a structural or fabrication standpoint, however, there is no improvement at all—the extra division adds a mold-seam at the perimeter rail, and neither disclosure addresses any of the well-known problems involved in molding the board's interior foam core.
In both inventions, the mold is used as a container and the liquid prefoam is poured into the mold cavity and allowed to rise parallel to the board's width—the foaming reaction of the polyurethane resin is deceptively simple, however; when complete, even the mixing cup appears to make a perfectly acceptable mold. The problem is that the foam's upward movement during expansion occurs before the resin begins to harden—the movement destroys the cellular structure of the foam against the interior surface of the board's fiberglass skin and concentrates the released blowing agent or gas in the same area, and leaves large areas of the fiberglass skin with a very thin, nearly continuous void directly beneath the surface, and little or no skin-to-interior core bond.
In the disclosure to Rubio, et al. the voids are identified, but the inventors incorrectly attribute the soft spots [i.e., the voids beneath the skin] to the expansion and contraction cycle of the foam (the soft spots are described as areas where the foam has pulled away from the resin in the fiberglass skin). They therefore suggest using a baffle to contain the expansion of the foam to force it against the interior surface of the skin—a partial step towards the prior art method of containing the foam in a high-strength mold and hydraulic press. Neither invention has seen production, since prior art problems of molding the board's interior foam core, the length of the mold-cycle, and the weak or inadequate joint between the board's two opposing sides are neither noted nor addressed. In known methods of sailboard production, the latter two problems are “solved” by eliminating the reinforcing fiber or foaming the resin matrix in the skin, and therefore entail a major reduction in strength.
In low-cost methods of sailboard production, for example, blow-molding or rotationally molding techniques are used to blow or melt a thermoplastic resin to the surface of the mold; this produces a continuous one-piece skin, but relegates production to beginner and entry-level sailboards due to the excess weight/inadequate strength caused by the lack of any composite material at all. The added drawback is that the interior foam core must be formed by injecting liquid pre-foam into the interior cavity of the closed mold, which involves the production problems outlined above.
Pre-molding the board's interior foam core is a very widely used alternative since it allows a major reduction in weight—the problem, as noted above, is the difficulty in consistently pre-molding the foam to tight tolerances (e.g., within several thousandths of an inch). In the prior art, the lack of close tolerances is compensated for by saturating the reinforcing fabric with an epoxy resin that has a blowing agent added; with the laminate/interior core assembly contained in a heated mold and a mechanical press, the epoxy expands to fill any voids or discrepancies between the pre-molded interior foam core and the closed cavity of the mold. With the added stiffness of a PVC sheet foam layer sandwiched between layers of foamed laminate in the skin, the board is light in weight and the quickly-curing foamed resin and rapid mold-cycle makes production costs competitive with hand methods of surfboard or sailboard production (outlined in more detail below).
An illustration of the low production costs using the method is provided by the U.S. Pat. No. 4,713,032 to Frank, the specification of which is incorporated herein, in which the prior art foamed epoxy laminate—often referred to as a “thermal compression-set epoxy” due to the high pressure and temperature cure—is replaced with a quickly-setting, foamed polyurethane resin for a very rapid mold-cycle of about twenty minutes per board, and high production from the molding tool of as many as twenty-four boards per day.
Using either resin, the pressurized cure limits the shape of the board an exact duplicate, while the foamed laminate replaces high-strength structural layers in the skin—this leaves strength-to-weight and skin-to-interior core ratios well below expensive high-performance sailboards, which eliminate the blowing agent in the resin matrix to create a much higher strength “structural sandwich” or “cored composite” skin.
b. Structural Sandwich/ “Advanced Composite” Production
The structural sandwich is expensive to fabricate because of the lengthy mold-cycle—vacuum pressure is used to conform the skin core to the shape of the mold, and removes entrapped air and voids in the composite skin; to prevent any spring-back of the skin core the material remains in the mold under vacuum pressure for about two to three hours until the resin has completely cured. Adding to production problems caused by the lengthy cure is the difficulty removing excess resin from the skin—the mold's sharp, upraised edge contours tend to create a dam, and the addition of the sheet core layer creates a buffer that blunts the squeegee pressure on the interior layers of laminate against the surface of the mold; the vacuum-bagging procedure requires an airtight mold and seals the entire skin, and prevents any excess resin from escaping during cure. In addition, the stiffness of the skin core generally exceeds the pressure available with vacuum (14.7 psi) and prevents it from being conformed to the very sharp curvature at the board's perimeter rail; the sharp curvature at the rail compromises the design of the joint and, due to the excess resin, the strength of the entire board as well.
The U.S. Pat. No. 4,964,825 to Paccoret, et al. illustrates the problems outlined above; it reveals a large gap in the sandwich structure and very poor joint design at the board's perimeter edge (a conventional inward-turning bonding flange is depicted)—the mold's sharp edge contours, the sharply inward curving bonding flange, and the complete seal of the vacuum-bag prevents any of the excess resin from escaping prior to cure. The invention is directed to improving the structural design of the board in the fin-box/mast track areas; the design of the mold-seam and the removal of excess resin are much larger problems, but neither is addressed.
The U.S. Pat. No. 5,023,042 to Efferding also reveals very poor joint design; complications in this case are due to the difficulty of laminating the sandwich skin to a very low-density, pre-molded EPS (expanded polystyrene) “bead” foam interior foam core. In the disclosure, the PVC sheet foam/wet epoxy laminate fits into molded-in recesses in the EPS interior foam core and the entire assembly is placed in the mold—the exterior surface of which precludes resin removal by hand. Vacuum pressure is used to press the components tightly together but exceeds the compressive strength of the foam, causing it to distort and crush, and also withdraws air trapped between the individual beads of foam—the “outgassing” of air from the foam results in pockets of entrapped air and large voids in the composite laminate.
To prevent these problems, Efferding suggests using a vacuum bore to withdraw air from the foam and discloses a novel female mold with a flexible perimeter portion that, under full vacuum, bulges outward evenly and allows the EPS interior core to assume an even, permanent compression set during cure. Structural compromises include the large gap in the sandwich skin structure at the board's perimeter rail, and the absence of internal spars, shear webs, or hollow, weight-reducing areas in the board's interior core—all of which would create distortion problems and/or prevent the board from compressing evenly during cure. The added problem is the high resin content in the skin—to draw vacuum the mold completely encases and seals the board structure and prevents excess resin from escaping during cure. A low temperature oven is also used to speed production, but costs are still very high due to the lengthy mold-cycle of just under three hours.
The inventor notes that known methods of sailboard production produce boards having a mold-seam at the point of greatest breadth thereof; the word “seamless” in the title of the invention refers to the modest improvement in the placement of the mold-seam—which is not in the expected location, but on the sharpest point of the rail.
Borrowing directly from the general mold configuration disclosed by Efferding, the U.S. Pat. No. 5,266,249 to Grimes III, et al., the specification of which is incorporated herein, teaches a method of forming interior joints in at least partially enclosed confined interior areas (see, e.g., the mold configuration to Smith) and an improved joint design as well—the composite layers meet and form an overlapping joint at the perimeter rail. The drawbacks are again caused by the mold's concave surface—fabrication of the joint requires a female mold and extremely costly “advanced composite” material; the tackiness of the partially cured “pre-preg” epoxy laminate is used to adhere the deck or top layers of the honeycomb core material to the walls of the mold; the bottom layers of board are then assembled on an inflated bag, which doubles as a vacuum bag and provides the very high outward pressure (i.e., at least 13 psi) needed to hold the composite skin/honeycomb core material in proper orientation and in pressurized contact with the mold prior to and throughout the cure.
According to the invention, there had previously been no method of applying fiber-reinforced plastic to the interior sides of mold-seams or joints; the inflated bag provides a very ineffective means for doing so, however, since its assembly adds considerable labor and limits the interior structure to a single support wall, rather than the higher strength and much lighter weight afforded by a plurality of internal shear webs or supporting struts. Further, neither surface (i.e., the inflated bag or the mold) provides the stability or the reference point required for the honeycomb core material to be accurately trimmed and glue—depoxy pre-preg core splice strips (strips of thermosetting epoxy that foam during the high-temp cure)—which must be held against the concave mold surface by the vacuum bag during cure—are therefore required to fill the gaps or voids around the perimeter rail where the honeycomb core cannot be accurately fit.
Although an obvious improvement over the prior art, the placement of the joint is at the sharpest point on the board's perimeter rail, and its design is less than ideal—the joint's shear and impact strength is reduced by the break in the fiber and the honeycomb core, causing earlier skin detachment and/or failure of the joint at lower levels of impact. Much greater impact resistance can be had by eliminating the joint itself: complete continuity of the core material would allow the continuity of the high-strength reinforcing fiber to be maintained throughout the perimeter edge, and reinforcement could also be confined to the interior side; much higher impact strength could also be had by increasing the density of the core material throughout the entire exposed perimeter rail area itself.
More importantly, the configuration of the mold makes it impossible to move the primary division between the two halves to the axis of symmetry; combining the joint with the support wall would create a far stronger board structure, since the support wall would provide internal reinforcement and an entire backup structure as well, in an area that is flat and only rarely exposed to high-point and impact loads.
As in the invention to Efferding, the mold completely encases and seals the board structure (bolts are depicted) and prevents any excess resin from escaping during cure; as noted above, an optimum fiber/resin ratio in the composite is extremely important—because the strength of the reinforcing fiber is usually several orders of magnitude higher than the resin (e.g., in a fiberglass composite, the tensile strength of glass, at roughly 500,00 psi, is roughly fifty times that of the resin, at 9-12,000 psi), excess resin in the composite typically causes a large reduction in strength. In sandwich skin fabrication, reducing the percentage of resin from the sixty to seventy percent range (by weight, and typical when the reinforcing fabric is saturated by hand) to the thirty-five percent level will usually double the compressive and flexural strength of the composite facing in bending—equally important, the weight saved can be used to increase the density of the stiff, lightweight sandwiched core; because the improvement in strength that comes by increasing the density of the core is not linear—i.e., doubling the core material's density will usually triple its compressive strength—an optimum fiber/resin ratio in the laminate can more than double the flexural, compressive and impact strength of the structural sandwich skin as a whole.
Reducing the resin content, however, requires special unidirectional fabrics or very tightly woven, difficult to saturate “crow-foot” or “satin” weaves of cloth, as well as a method for applying fairly high pressure to physically force the resin from the fiber, a lack of obstructions to allow the resin to actually be removed, and a barrier (e.g., a thin plastic film) to prevent air from re-entering the laminate (due to the slight spring-back of the fiber) once the pressure has passed. With current methods of production, efforts to employ these techniques have been less than completely successful due to the stiffness of the core, and the shape of the board and/or mold, as outlined above—Paccoret et al. and Grimes III et al. therefore teach the use of extremely expensive “pre-preg” or “advanced composite” material to keep the resin content to an absolute minimum.
As the name suggests, in the “pre-preg” the reinforcing fabric is pre-impregnated with the precise amount of epoxy resin (by Hexcel, Ciba-Geigy etc.), the resin is then “B-staged” or partially cured, the material is shipped under refrigeration to the end-user (usually large airframe manufacturers such as Boeing etc.), the material is then placed in the mold and undergoes a high-temperature, high-pressure autoclave cure. Due to the prohibitively high material cost, the lengthy two to three hour mold-cycle (½ hr. to heat, 1-1½ hr. cure, ½ hr. to cool) and the high-temp, pressurized cure, the “advanced composite” or pre-preg laminate/honeycomb skin boards occupy only a small niche in the overall sailboard market.
The further drawback is that the generally hollow board structure is best used on very thick sailboards—where the higher overall volume of the interior foam core adds a great deal of weight but little strength—and a foam core offers lighter weight with thinner, high-performance wave-boards (sailboards) and surfboards. The molds and methods outlined above, however, are not interchangeable—the high-temperature and pressurized autoclave cure needed for the honeycomb core/pre-preg laminate (e.g., 250° F. and a minimum pressure of 13 psi) typically exceeds the compressive strength of very low-density foams, and will melt polystyrene based foams (specified by Efferding, for example); specially designed molds are also required for the mechanical/hydraulic press involved in attaining adequate adhesion using liquid resin pre-foams, whether the material is used as a fiber-reinforced foamed laminate in the skin (e.g., the invention to Frank) or as the board's interior foam core. Hence, in the prior art, the configuration of the board and the materials used in construction are not readily optimized to the design of the board or performance requirements of the rider; compounding the problem, the mold's concave, female surface defines the board's exterior shape and restricts production to a series of exact duplicates.
Because of the very light weight and lower production costs possible when the board is fabricated by hand, molded surfboard production has been virtually nil since the beginning of the “modern era,” which began with the introduction of moldable plastic foam and fiberglass-reinforced plastic over four and five decades ago respectively, while custom “one-off” sailboards comprise a very significant portion of the overall market—particularly in high performance area such as Hawaii. In surfboard and high-performance sailboard production, the wide range of size and shape requires a large and prohibitively expensive inventory of molds, and eliminates the many custom design modifications that are now made as a matter of routine—the concave configuration of prior art female molds prevents the board's width, planing area and lengthwise “rocker” curvature from being tailored to the size of the waves or the individualized requirements of the rider.
C. Custom or “One-Off” Board Production
In custom or “one-off” (surf)board production, the board is individually hand-shaped from a polyurethane foam “blank;” the fiberglass and resin are then applied by hand over the shaped foam core. The process is labor-intensive and requires considerable skill, but the problems of molded manufacture are limited to a pre-production phase—the board's interior foam core is first molded by a separate manufacturer into a rough surfboard-shaped slab of foam, before being shipped to the surfboard manufacturer to be used in the actual construction.
To enhance strength and better control the somewhat unreliable reaction of the low-density polyurethane foam, the blank is molded in an extremely strong, heavy mold made of reinforced concrete. This allows an excess of liquid pre-foam to be poured in the mold; as the foam expands, the excess compresses under high pressure against the surface of the mold and produces a density-gradient in the blank—the foam is soft and weak in the center and becomes progressively harder and denser towards the surface. To avoid removing too much of the harder, denser surface foam during shaping, the blank is molded close-to-shape, or as thin as possible. The close-to-shape molding increases the already very large number of blank molds required for surfboard production, and frequently leaves the shaper with insufficient foam for the proper thickness or the required lengthwise bottom curvature or “rocker” on the board.
The molded-in rocker of the blank must therefore modified by the blank manufacturer—the blank is cut in half lengthwise, and the two halves are glued to a wooden center spar or “stringer” cut to the rocker curvature specified by the customer, and usually selected from a list of stock lengthwise rocker modifications. Clark Foam of Laguna Niguel, Calif., (www.clarkfoam.com) provides a Rocker Catalog listing the dimensions of over two thousand different templates available to modify the molded-in rocker curvature of the more than sixty different blank molds offered for surfboard production. With shipping and inventory problems at both ends of production, manufacture of the blank is expensive, but essential, since it allows the board to be tailored to the according to the flotation, planing area, and performance requirements of the rider.
After shaping, the fiberglass laminate is applied directly to the shaped foam, which provides a smoothly curving convex surface. With a fiber-reinforced composite, a convex substrate provides the foundation for a stronger, lighter structure—excess resin is easily removed for higher strength and lighter weight, and joint creation is stronger and simplified—the fabric can be pulled taut and a double overlapping joint created to provide a protective covering for the very exposed perimeter edge or “rail” and the very sharp convex curvature at the nose and tail as well.
The drawbacks include the large amount of labor and extra coats of resin required to sand the overlapped area completely smooth, and the board's very light weight—for higher performance, overall board weight has been consistently reduced to the point where the low-density interior foam core is no longer strong enough to fully support the board's thin exterior skin. The single fiberglass ply used on the bottom of the board will usually dent or fracture with moderate finger/thumbnail pressure, while the double or triple layer on the deck (or top surface of the board) that reinforces the tail area where the rider stands often fatigues, becomes permeable to water, then fails and completely delaminates under the repeated high pressure of the rider turning the board. Hand-shaping also limits the effectiveness of the longitudinal reinforcement—it makes wood the material of choice for the center spar and also makes it impractical to add top and bottom spar caps (i.e. the top and bottom reinforcing flanges in an I-beam)—the lack of effective longitudinal reinforcement leaves thinner surfboards in particular susceptible to breakage.
With current methods of production, the strength of the “one-off” or custom board is severely compromised by the roughly one-to-one weight ratio between the fiberglass skin and interior foam core—efforts to alter this ratio have been largely unsuccessful. The U.S. Pat. No. 5,569,420 to Van Horne, for example, suggests increasing the density of the polyurethane foam core—this is done by laying out sequential lines of liquid pre-foam that expand and, with the exposure to air slowing the reaction of the foam, leaves the foam with a hardened arcuate shell on its exterior surface; the procedure is repeated until a billet is formed; the foam for the board's interior foam core is then cut from the billet and hand-shaped the final dimensions. The procedure increases the overall density of the foam, but eliminates one of the primary advantages of the one-off method of production, which is the extremely rapid mold-cycle in the molding, of the blank (e.g., in the invention to Frank, a reaction retarder is needed to extend the rapid five minute setting time of the polyurethane resin in the foamed, fiber-reinforced plastic skin).
The U.S. Pat. No. 4,255,221 to Young teaches a laminated plywood skin created from individual layers of veneer, which are conformed to the curvature of a hand-shaped interior foam core using vacuum pressure. To reduce weight, Young provides additional adjustable means outside the vacuum forming apparatus to squeeze excess epoxy from the layers and to aid in conforming the wood to the hand-shaped interior foam core. The difficulty is in forming an effective joint at the perimeter rail—since the veneer can break if the curvature is severe, the edge contours of the board are made by laminating strips of wood directly to the interior foam core; after curing, the strips are hand-planed to the final dimensions and form a solid laminated wood perimeter rail.
In the disclosure to Efferding (discussed in greater detail above), the inventor describes developing a similar vacuum-bagging method for fabricating a sandwich skin sailboard. The use of composite material greatly complicates the process, however—for example, no attempt is made to conform the high-density PVC sheet foam to the sharp curvature at the perimeter rail, since it will break well before it reaches a right angled bend; in areas of very sharp curvature, its stiffness typically exceeds the pressure available with vacuum (i.e, 14.7 psi) and the compressive strength of the low-density EPS interior foam core as well. The further problem is the lack of any reference point along the board's perimeter edge—because of the difficulty trimming the composite core material and the gap or mismatch between the board's two opposing sides, the inventor teaches that the core material should fit into recesses in the interior foam core. Efferding reports that it takes between thirty to forty five hours to manufacture an acceptable sailboard using the technique; to reduce labor, Efferding discloses and teaches the use of a novel female mold.
In “one-off” or custom production, the board's barely adequate level of strength is due to the lack of a strong, composite based skin structure that can be fabricated using a convex surface such as plastic foam; in molded manufacture; the high production costs can be traced to the fact that the stiff, lightweight composite core materials and reinforcing fabrics are not suited to being fabricated in the concave cavity of a mold at all, particularly when the inward turning surface performs a shape-defining function, as is the case with prior art female molds. With either method, the primary problem is the inability to conform the stiff, lightweight composite core materials and reinforcing fabrics to the sharp compound curvature at the board's perimeter edge and rail.